1. Field
The present invention relates to a novel process for manufacturing polyene esters and acids from acetalized polyene aldehydes via an acid-catalyzed condensation reaction with vinylketene acetals.
2. Description
Lewis acid-catalyzed additions of .alpha.,.beta.-unsaturated ethers (enol ethers) to acetals have been known for a long time and date back to the work of Muller-Cunradi and Pieroh (see U.S. Pat. No. 2,165,962). Hoaglin and Hirsch [J.A.C.S. 71: 3468 (1949)] investigated this reaction further and broadened the possible applications, which Isler et al. likewise did in the 1950's with respect to the synthesis of .beta.-carotene, crocetin dialdehyde, lycopene as well as .beta.-apocarotenoids [see Helv. Chim. Acta, 39: 249 et seq. and 463 et seq. (1956), ibid., 42: 854 et seq. (1959) as well as U.S. Pat. Nos. 2,827,481 and 2,827,482]. Later, Mukaiyama [Angew. Chem., 89: 858 et seq. (1977) and Org. Reactions, 28: 203 et seq. (1982)] extended the reaction by using the readily accessible trimethylsilyl enol ethers.
The first Lewis acid-catalyzed condensations of 1-alkoxy-1,3-dienes (dienol ethers) with .alpha.,.beta.-unsaturated acetals were reported by Nazarov and Krasnaya [J. Gen. Chem. USSR, 28: 2477 et seq. (1958)] and by Makin [Pure & Appl. Chem., 47: 173 et seq. (1976), J.
Gen. Chem. USSR, 31: 3096 et seq. (1961) and 32: 3112 et seq. (1962)]. Here, the coupling of the acetal to the dienol ether takes place as far as can be seen exclusively at its .gamma.-position with the formation of a chain-lengthened .alpha.,.beta.-unsaturated acetal, which, however, in competition with the first acetal reacts with further dienol ether to form a further, chain-lengthened .alpha.,.beta.-unsaturated acetal etc. [telomer formation; see also Chemla et al., Bull. Soc. Chim. Fr., 130: 200 et seq. (1993)]. For this reason such a condensation has been found unworkable for synthetic purposes, especially for the synthesis of apocarotenoids [Isler et al., Adv. Org. Chem., 4: 115 et seq. (1963)].
Not only 1-alkoxy-1,3-dienes, but also trimethylsilyloxydienes [of the type CH.sub.2 .dbd.CH--CH.dbd.CH--OSi(CH.sub.3).sub.3 ] can be condensed with acetals in the presence of Lewis acid catalysts, as disclosed by Mukaiyama et al. in Chem. Lett. 1975, 319 et seq. In this coupling too the attack takes place exclusively at the terminal (.gamma.-) carbon atom of the diene system in order to form ".gamma.-products" [see Mukaiyama et al., Bull. Chem. Soc. Jap, 50: 1161 et seq. (1977) and Japanese Patent Publication (Kokai) 36,645/1977]. In contrast to the reaction with 1-alkoxy-1,3-dienes, in the case of the reaction of trimethylsilyloxydienes with acetals there is formed an aldehyde which does not react further with the diene (no telomer formation). By using this method Mukaiyama et al. were able to synthesize vitamin A [see Kokai, 36,645/1977, Chem. Lett. 1975, 1201 et seq. and Bull. Chem. Soc. Japan, 51: 2077 et seq. (1978)] and workers from Rhone-Poulenc developed new routes to carotenoids and vitamin A (see DOS 2,701,489 and A.E.C. Societe de Chimie Organique et Biologique No. 7824350).
Silylated vinylketene acetals [of the type CH.sub.2 .dbd.CH--CH.dbd.C-(Oalkyl)(OSi(CH.sub.3).sub.3)] can also react with acetals in a manner analogous to the aforementioned trimethylsilyloxydienes [see Tetr. Lett., 20: 3209 et seq. (1979) and Chimia, 34: 265 et seq. (1980)]. As evident from, among others, Tetr. Lett., 22: 2833 et seq. (1981), ibid. 26: 397 et seq. (1985), DOS 3,244,273 and U.S. Pat. No. 4,937,308, the known reactions always formed not readily separable mixtures of the two possible .gamma.- and .alpha.-coupling products [".gamma.-products" . . . CH(Oalkyl.sup.1)--CH.sub.2 --CH.dbd.CH--COOalkyl.sup.2 ; ".alpha.-products" . . . CH(Oalkyl.sup.1)--C(CH.sub.3)(CH=CH.sub.2)--COOalkyl.sup.2 ], rendering this reaction--at best marginally usable for synthetic purposes in the carotenoid field. This reaction would be interesting and useful only if complete .gamma.-selectivity could be achieved, for example for the synthesis of polyenes, namely apoesters, crocetin esters etc.; because by elimination of the alcohol alkyl.sup.1 OH from the .gamma.-product it is possible to form, if desired, a further (conjugated) double bond with the formation of the product . . . CH.dbd.CH--CH.dbd.CH--COOalkyl.sup.2. Thus, such polyenes could be produced without employing the Wittig or Horner reaction hitherto used for this purpose.